Hi Alun,
Unwanted movement of components is a significant issue in spectrographs and the the tolerances are very small. (1/5 pixel (~1um) shifts are easily detectable.) The LHIRES design is particularly prone to these problems. Have you checked by aiming the spectrograph in different directions (NSEW,Zenith) and watching the position of the lamp lines that your mod is not introducing any additional unwanted instability ?
Cheers
Robin